Experienced craftspeople spend a good deal of time sharpening their tools, since the best results are generally obtained with very sharp tools, and sharp edges are rapidly blunted when the tools are in use.
If the craftsperson is skilled at sharpening these tools, only a relatively few strokes of the tool on a sharpening stone are needed to sharpen it, particularly if the tool has a primary sharpening angle and, at its very tip, a micro-edge at an angle somewhat higher than the primary angle. However, even an experienced craftsperson gains by using a jig or other sharpening device to steady the tool and hold it at the correct angle to the stone, since a hand-held tool tends to rock during sharpening with the result that metal is unnecessarily removed and the sharpening operation sometimes becomes problematic and unduly time-taking.
It is desirable to conserve the metal of the tool since the blade will otherwise be prematurely shortened, making it eventually unfit for its intended purpose, or, if the blade is tapered along its full length, reaching a stage where its working end is excessively thick. Efficiency in sharpening is therefore important.
It is crucial that there be a high degree of consistency in the way in which a tool is sharpened, so that, even if the angle to which it is sharpened is not perfectly correct, the angle is precisely replicated each time the tool is sharpened.
Sharpening angles are generally within the range 15 degrees to 60 degrees, measured as the included angle between the rear face of the tool (which for present purposes will be regarded as the face on which the sharp edge is provided) and the tapered zone at the tip of the blade. The front face will for present purposes be regarded as the face from which the tapered zone extends at an oblique angle towards the sharp edge of the tool. If the blade has a micro-edge, this edge extends from the rear face of the tool a short distance up the taper of the tip.
If a tool is short and the sharpening angle is in the lower part of the range, below about 30 degrees, access to its front face during sharpening is limited, and only compact structures can be accommodated in the space between the surface of the stone and the front face of the tool. This imposes a significant challenge to the designer of sharpening devices.
Moreover, since workshop space is usually limited, it is usually only possible to allocate a relatively small space on a bench or other work-top to the sharpening of tools. It is consequently desirable to have a single sharpening device for all the relevant tools in a workshop, capable of working with both low and high sharpening angles and with both flat and tapered blades, and which occupies only a minimal space. It is also important that the procedures involved in using the sharpening device be as brief and simple as possible, so that efficient use of workshop time is promoted.
While many sharpening devices have been proposed, only a few have had enduring success on the market. One of these, described in U.S. Pat. No. 4,733,501 (McLean), is in two parts. The first part comprises a cylindrical roller which in use runs backwards and forwards along a sharpening stone and which rotates about a shaft to which is attached a blade-holder including a flat clamping plate and a hand-screw mounted in a bracket located above the clamping plate. The hand-screw is used to engage the rear face of the blade and clamp the front face of the tool on to the clamping plate. The sharpening angle is set by aligning the rear face of the blade with one of the planar faces of a multi-faced block which is clamped by a further hand-screw to a base plate on which the setting up of the blade in the device takes place. The base plate and the multi-faced block mounted on it make up the second part of the device. The block, when loosened from the base plate by use of the hand-screw on this component, is rotatable between different positions on the base plate to expose different faces which make different included angles with the plane of the base plate. These angles are chosen to match the most common sharpening angles of planes and chisels, so that the device can accommodate a number of different angular settings for blades to be sharpened. Once the block has been set and locked in the desired position on the base place, the blade is positioned against the appropriate face of the block, in full surface-to-surface contact with it, with the tip of the blade engaging the base plate. The other part of the tool is then brought into position on the base plate, the hand-screw on that component having been loosened for this purpose. In the final position the roller rests on the base plate and the front face of the blade engages the clamping plate. The hand-screw is then tightened so that the blade is fixed relative to the clamping plate and consequently to the roller. The blade, with the first part of the device attached in this fashion, is then moved on to a sharpening stone, such as a whetstone or an oilstone, and is pressed on to it while the blade and the roller are made to run backwards and forwards on its surface until sharpening has been achieved. The mountings of the roller on the first part of the device include an eccentric mechanism to allow the roller to be moved relative to the blade to a position where the tool assumes a greater included angle to the plane of the stone than the original angle to which the device had been set, so that a micro-tip can be ground on to the blade by further rolling action.
While the device of U.S. Pat. No. 4,733,501 is considerably simpler than many other sharpening devices which have been proposed, it suffers from the disadvantage that the setting up operation, involving the clamping of the block on the base plate at one of the angles defined by its various faces and then positioning the blade alongside this face before bringing the other part of the device into operation, is both somewhat time-taking and error-prone, since it can usually be done with only one hand, the operator's second hand being occupied with mounting the blade in the first part of the device and then clamping it on to the clamping plate. The number of sharpening angles which the device can provide is also limited to the number of faces on the multi-faced block.
Other known sharpening devices tend to be considerably more complicated. For example, U.S. Pat. No. 5,582,542 (Stein) discloses a device which incorporates a base on which a sharpening stone is moved backwards and forwards, or sideways, relative to a blade which is clamped in a holder which is mounted on the base. The holder comprises an arm with an end part that is articulated to the rest of the arm, the blade being clamped to this end part with its rear face uppermost. The arm is rotated into the selected position and then locked in this position for the sharpening operation. This arrangement leaves the tapered surface of the blade located parallel to the plane of the base plate, so that the movement of the stone on the base plate provides an abrading action on the tapered zone of the blade. No guidance is provided as to how to set up the device for any particular sharpening angle, and in practice that task must be undertaken by the use of another tool, such as a protractor. The device as a whole is relatively bulky and takes up a considerable space on a workbench or elsewhere in a workshop.
Another relatively bulky and complicated sharpening device is described in U.S. Pat. No. 4,217,735 (McGeoch), where a blade to be sharpened is also clamped relative to a fixed superstructure and a sharpening stone is reciprocated underneath the blade to abrade its tapered tip.
Thus, a need exists for an improved apparatus and method to provide a sharpening device for plane blades, chisel blades (whether flat or tapered), and the blades of similar tools, which is compact and simple to operate and allows quick and accurate selection of any sharpening angle within the normal range for this type of tool. It is believed that the present invention addresses these and other needs.